The title compounds are mercaptosilane derivatives in which the mercapto group is blocked, i.e., the mercapto hydrogen is replaced by another group, the so-called blocking group. Of specific interest are thioester-containing organosilicon compounds that can be prepared by reacting a mercaptosilane with organic or inorganic halides or anhydrides, and in particular thiocarboxylic organosilicon compounds that have been identified as suitable coupling agents in the manufacture of silica-filled rubber mixtures (U.S. Pat. No. 6,414,061). Thiocarboxylic silanes are prepared from mercaptosilanes and preferably acid chlorides. The by-product in this reaction, anhydrous hydrogen chloride, triggers a number of unfavorable side reactions, including the transesterification of ethoxysilanes to mixed chloroethoxysilanes or even chlorosilanes. These reactions are very fast and cannot be prevented by manipulating common process parameters, e.g., temperature or pressure. It is possible, but technically very difficult, to restore the Si—OEt group (by means of a neutralization procedure). A much better way to run these reactions is to in situ neutralize HCI on a molecular level by means of an acid acceptor. This reaction is faster than the transesterification reaction and leads to very little formation of Si—Cl groups. Challenges that need to be met with this technology are salt handling and environmentally friendly recycle operations.
The step of scavenging an acidic by-product with a base is well known. Tertiary amines are commonly used, among these triethylamine is cited most often in the patent literature. Triethylamine is the least costly tertiary amine, but its most economical use requires a filtration step. A stoichiometric amount of triethylamine hydrochloride must be separated from the product. This is a mechanically intense unit operation and usually leads to poor yields if the product in the filter cake is not recovered. Either way, in addition to the already costly filtration or centrifugation step, additional costs relating to the disposal or further processing of the filter cake are added. Moreover, trialkylamine hydrochlorides are difficult to filter and typically require special 0.1 to 0.01 μm pressure filters.
Alternatively, the mercaptosilane can be reacted with an acid chloride in the absence of any amine. In this case, mixed chloroethoxysilanes are formed by relatively fast side reactions. Theoretically, the formation of these chloroethoxysilanes can be prevented by efficient removal of HCl. However, due to its covalent nature, anhydrous hydrogen chloride has a significant solubility in almost any non-polar organic medium and turns out to be very difficult to remove. In fact, almost any binary system containing HCl behaves in a highly non-ideal manner. Prior art includes specially designed contactors, e.g. a falling film reactor, a Couette reactor, a rotating disk contactor, etc., that are designed to facilitate removal of a gaseous species. However, efficient removal of highly reactive species is very difficult. In the case of HCl removal it may not be facilitated to a level where transesterification to chloroethoxysilanes is low enough to obtain a product that contains only trace amounts of these molecules.
It is possible, although technically difficult, to neutralize chloroethoxysilanes with neutralizing agents. The highly reactive thiocarboxylate group enhances this difficulty, particularly when basic inorganic neutralizing agents are used.
Another alternative technology is to react mercaptosilanes with alkali metals to yield the alkali organothiolates prior to the main reaction with the acid chloride. Hence, neutral sodium chloride is being formed as by-product.
The concept of scavenging a highly reactive acidic by-product like anhydrous hydrogen chloride with a tertiary amine is generally known as a synthesis technique.
U.S. Pat. No. 6,229,036 B1 describes the amine-assisted addition of a chlorosilane to a mercapto silane to produce sulfanylsilanes. Triethylamine is explicitly mentioned in the examples. Removal of the corresponding amine hydrochloride salt is achieved by filtration. The amine hydrochloride salt cannot be separated by a water/brine wash since silyl-blocked mercaptans hydrolyze in water. For example, U.S. Pat. No. 6,147,242 describes the preparation of silylalkylthiols by reacting silylalkylsulfanylsilanes with water. Hence, it is demonstrated that the Si—S bond is hydrolyzed faster than Si—OEt bonds.
U.S. 2001/00556139 B1 and EP 1 142 896 A1 describe the reaction between triazine compounds containing functional groups (e.g. chlorine) and mercaptosilanes to yield sulfur silane-triazine derivatives. A relevant example is included: The amine (triethylamine) assisted addition of mercaptopropyltriethoxysilane to cyanuric chloride (2,4,6-trichloro-1,3,5-triazine). Separation of the triethylamine hydrochloride salt from the product (1,3,5-tris(mercaptopropyltriethoxysilyl)triazine) is achieved through filtration. Triethylamine is the only amine that is explicitly mentioned.
U.S. Pat. No. 6,414,061 B1 describes novel blocked mercaptosilanes and includes methods of preparation. In Example 9 of the patent, the preparation of 3-(octanoylthio)-1-propyltriethoxysilane is described using triethylamine as the acid scavenger. Separation of triethylamine hydrochloride from the product was achieved through filtration (two times: first through a 0.1 μm filter, and then through a 0.01 μm filter).
The use of an aqueous phase in the production of polysulfidic silanes, such as bis(triethoxysilylpropyl) tetrasulfide and the corresponding disulfide, two articles of commerce, is prior art. The aqueous phase can be present during the reaction (phase transfer catalyzed reaction of chloropropyltriethoxysilane with water soluble M2Sn and/or MSH and/or elemental sulfur (M is ammonium or alkali metal) or can be introduced after completion of reaction to separate the product from the reaction mixture.
U.S. Pat. No. 5,405,985 describes the manufacture of molecules of the formula Z-Alk-Sn-Alk-Z. A compound of the formula Z-Alk-X, e.g. chloropropyltriethoxysilane, is reacted with ammonium or alkali polysulfide in the presence of an aqueous phase and a phase transfer catalyst. The by-product of the reaction, ammonium or alkali halide, most commonly sodium chloride, stays in the aqueous phase after completion of reaction.
U.S. Pat. No. 6,294,683 B1 describes the preparation of sulfur-containing organosilicon compounds by reacting aqueous solutions of various polysulfidic anions in saturated sodium chloride brine solutions with chloropropyltriethoxysilane (CPTES) supported on carbon black in the presence of a phase transfer catalyst.
U.S. Pat. No. 6,384,255 B1 describes the preparation of molecules of the formula Z-Alk-Sn-Alk-Z by phase transfer catalysis techniques. The phase transfer catalyst, elemental sulfur, and sulfide compounds of the formula M2Sn or MHS (M is ammonium or an alkali metal) are mixed in water and allowed to react to an intermediate reaction product. In a second step, this intermediate reaction product is reacted with an organosilane, preferably chloropropyltriethoxysilane (CPTES). The process requires a filtration step to remove residual sulfides from the organic phase.
U.S. Pat. No. 6,384,256 B1 differs from U.S. Pat. No. 6,384,255 B1 in the sense that in the first reaction step, an alkali metal hydroxide compound is reacted with a sulfide compound of the formula M2Sn or MHS (M is ammonium or an alkali metal), and elemental sulfur in water to form a polysulfide mixture which is then reacted with CPTES in the presence of a phase transfer catalyst. The pH of the aqueous phase is adjusted using a buffer. According to the patent, this process minimizes or eliminates hydrogen sulfide as a side product. It requires a filtration step to remove residual sulfides from the organic phase.
In U.S. Pat. No. 6,448,426 B1, a very similar reaction is described as in U.S. Pat. No. 6,384,255 B1. This patent also addresses the separation of the sulfur-containing organosilicon compounds from the product mixture by adding water or a dilute acidic solution to the product mixture, and phase separating the product mixture into an organic phase that contains the product and into an aqueous phase that contains ionic polysulfides.